The present invention relates to transport and storage of compressed video and audio streams. Illustratively, the invention is illustrated herein using the International Standards Organization (ISO) Motion Picture Experts Group (MPEG) 2 standard for producing compressed video, storing the compressed video and transporting the compressed video to a decoder. See ISO/IEC IS 13818-2: Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Video; ISO/IEC DIS 13818-1: Information Technology-Generic Coding of Moving Pictures and Associated Audio Information: Systems. For compressed audio there are numerous standards including ISO.backslash.IEC IS 11172-2: 1993 Information Technology-Coding of Moving Pictures and Associated Audio for Digital Storage Media at up to about 1.5 Mbit/sec-Part 3 Audio (MPEG-1 Audio); Dolby AC-3; ISO.backslash.IEC DIS 13818-3: 1994 Information Technology-Coding of Moving Pictures and Associated Audio Information: Audio (MPEG-2 Audio). The contents of the ISO documents are incorporated herein by reference.
Illustratively, MPEG-1 and MPEG-2 provide for hierarchically organized streams. That is, for a given program, audio and video streams are separately encoded to produced compressed audio and video streams. These compressed audio and video streams are illustratively placed in a higher layer stream such as an MPEG-2 compliant program or transport stream. Closed caption text or other private data of the program may also be placed in the higher layer stream. Each program may include multiple video streams (e.g., multiple camera views) and multiple audio streams (e.g., different language audio). The higher layer transport or program stream provides a manner for associating all related encoded video, audio and private data streams so that they can be extracted, decoded and presented together in a coherent fashion. Furthermore, the higher layer stream may include compressed audio, video and private data for only a single program or for multiple programs. The higher layer stream (i.e., program or transport stream) may be stored in a storage device such as a digital video disc (DVD), video tape, magnetic disk drive, etc. Alternatively, the higher layer stream (i.e., transport stream) is transmitted via a transmission channel. Before transmitting or storing the higher layer transport or program stream, the program or transport stream may be encapsulated in an even higher layer storage format or channel layer stream.
The invention is illustrated herein for transport streams which are transmitted via a transmission channel. However, the scope of the discussion should be sufficiently general for application to program streams and to recording and reproduction of program or transport streams using a storage device.
FIG. 1 depicts an illustrative communications network which illustrates the hierarchical aspect of encoding and decoding. An audio-video signal, such as an analog NTSC, PAL or HDTV signal is divided into audio and video component signals. The audio signal is digitized and encoded by an audio encoder 54. The video signal is converted to SIF format (i.e., luminance signal Y and chrominance signals C.sub.r and C.sub.b), digitized and inputted to a video encoder 52. The compressed audio and video outputted from the audio and video encoders 54 and 52 are inputted to a transport stream encoder/multiplexer 56. A private data signal (such as closed captioned text) may also be inputted to the transport stream encoder/multiplexer 56. Such compressed video, compressed audio and private data signals are referred to as elementary streams. The transport stream encoder/multiplexer 56 illustratively also receives elementary streams for a number of other programs. The transport stream encoder/multiplexer 56 multiplexes the elementary streams of one or more programs into one or more transport streams. The transport streams outputted by the transport stream encoder/multiplexer 56 are inputted to a channel encoder 58. The channel encoder 58 encapsulates the one or more transport streams into one or more channel layer streams. The channel layer streams outputted by the channel encoder 58 are then transmitted via a transmission channel 60. The transmission channel 60 may be a telephone network, a cable television network, a computer data network, a terrestrial broadcast system, or some combination thereof. As such, the transmission channel may include RF transmitters, satellite transponders, optical fibers, coaxial cables, unshielded twisted pairs of wires, switches, in-line amplifiers, etc.
The transmitted channel streams are received at a channel decoder 68. The channel decoder 68 recovers the one or more transport streams from the received channel streams. The recovered transport streams are then inputted to a transport stream decoder/demultiplexer 66. The transport stream decoder/demultiplexer 66 extracts particular elementary streams from the inputted transport streams corresponding to one or more user selected programs. An extracted video signal elementary stream is inputted to a video decoder 62 and an extracted audio signal elementary stream is inputted to an audio decoder 64. The video decoder 62 decodes the video signal elementary stream and outputs a decompressed video signal. The audio decoder 64 decodes the audio signal elementary stream and outputs a decompressed audio signal. Illustratively, the decompressed video signal, the decompressed audio signal and private data signal may be combined to produce an NTSC, PAL or HDTV signal.
To better understand the considerations associated with decoding hierarchical encoded streams of audio and video, the elementary and transport stream layers are now discussed in greater detail. While channel streams are not specifically discussed, a variety of standard channel streams have been proposed for transmission via different kinds of channels. See J. PROAKIS, DIGITAL COMMUNICATIONS, 2nd ed. (1989) for a discussion of forward error correction of errors which may arise in the channel.